The invention relates to a laser nozzle that can be used in laser-beam cutting with an internal mobile element including a skirt for concentrating the cutting gas in the cutting groove, that can be better implemented industrially and has an improved service life.
Laser-beam cutting requires the use of a nozzle, usually made of copper, that is used to channel the gas while enabling the laser beam to pass through.
The diameter of the output orifices of the nozzles is typically between 0.5 mm and 3 mm for a working distance of between 0.6 mm and 2 mm.
High pressures, usually several bar, are required in the focusing head for cutting to enable the gas to enter the groove and to remove the molten metal.
A significant part of the gas used, typically between 50% and 90%, has no effect on the cutting process, i.e. on the expulsion of the molten metal, since it flows onto the sides of the cutting groove.
These gas losses are in fact attributable to the huge difference between the area of the valve orifice and the size of the focal spot. By way of example, the area of a nozzle with an output orifice having a diameter of 1.5 mm is 25 times larger than the area of the focal spot created by the laser beam passing through this nozzle.
If an insufficient proportion of gas is used, cutting defects may appear, in particular attached burrs and/or traces of oxidation.
Attempting to resolve this by reducing the diameter of the orifice of the nozzle is not ideal since there is then a risk of the laser beam hitting the inside of the nozzle and damaging it, which would also adversely affect the quality of the cut and/or performance levels.
There are a number of documents proposing different solutions to help the gas to enter the groove, for example EP-A-1669159, JP-A-62006790, JP-A-61037393, JP-A-63108992, JP-A-63040695 and U.S. Pat. No. 4,031,351.
None of these solutions is ideal since the architecture is often complex to implement, they are more bulky than a conventional nozzle and/or they provide limited efficiency.
Notably, document U.S. Pat. No. 4,031,351 discloses a laser cutting nozzle including a mobile element, the extremity of which is pressed by a spring against the surface of the part to be cut to assist the injection of the cutting gas into the groove.
The key drawback to this solution is that the force exerted by the spring in the direction of the sheet, combined with the pressure of the cutting gas, causes the mobile element to exert a significant force on the sheet to be cut. This results in a risk of the sheet being deformed, scratched or even dragged, said sheet normally being simply placed on the table of the industrial cutting machine.
To overcome this, French patent 1154224, filed on May 16, 2011, proposes arranging a mobile element in the body of a laser nozzle. This mobile element can be moved axially within said body, under the effect of a gas pressure and towards the surface of the sheet to be cut. This causes the mobile element to approach the upper surface of the sheet to be cut, before it comes in contact with the sheet, thereby forming a skirt, the external profile of which is cylindrical, on account of which the cutting gas is channeled and concentrated on the groove, which forces the gas to enter the groove and improves the effectiveness thereof.
Furthermore, this nozzle includes an elastic element exerting an elastic return force on the mobile element in a direction tending to move it away from the sheet. Thus, when the gas is cut off, the mobile element can be returned to the idle position thereof and therefore the skirt moves back inside the nozzle body.
However, this solution has several drawbacks, in particular in terms of industrial application.
Indeed, different types of obstacles can be found on the surface of the sheet to be cut and can hinder the movement of the mobile element in the working position thereof, i.e. in contact with the sheet. For example, these may be overthicknesses of molten metal caused by projections occurring during the drilling and cut initiating phases, or parts already cut out that remain wedged in the sheet and positioned at an angle in relation to the surface of the sheet, i.e. that have a portion located above the level of the upper surface of the sheet. Cuts started from an edge of the sheet can also generate steps or height differences, i.e. differences in level between different portions of the sheet, on account of a defamation or lowering of certain portions of the sheet caused during the cut.
There is then a risk of stop-starting and impacts at the extremity of the skirt located facing the sheet if the peripheral wall of the skirt comes up against any obstacles present on the sheet. The movement of the skirt can be slowed, abruptly braked or even stopped.
If detected by the cutting machine, the impacts with the extremity of the skirt may cause untimely emergency stops of the machine. Such impacts could also damage and affect the efficiency of the skirt, even completely damaging or breaking it. It is then necessary to replace the mobile element of the nozzle, which increases production costs and reduces productivity of the industrial cutting machine.
These problems become even more significant when cutting using “nesting”, which is common in industry on account of the material savings achieved, for which the parts cut out from a single sheet are arranged very close to one another, and some parts can even have common edges.
The document JP-A-7251287 proposes a laser nozzle including a mobile element and a spring exerting a force tending to press said element against the sheet. Furthermore, this nozzle uses the force exerted by the cutting gas leaking beneath the surface of said element facing the sheet to be cut to cause a slight distancing of the movable element from the sheet.
However, this solution is difficult to implement since it requires precise control of the different forces in play to adjust and maintain the desired distance. Furthermore, this distancing reduces the effectiveness of the gas injection into the groove. It creates further problems during the steps of the method in which no cutting gas is supplied by the nozzle, or when only a low cutting gas pressure is supplied to the nozzle.
This is in particular the case when drilling the sheet, which is done at low gas pressures, typically less than 4 bar, or when the nozzle is moved quickly above the sheet, at a typical distance of 0.5 mm to several mm which is done without any cutting gas or beam, in particular when several nested parts are cut out of a single sheet. To improve production rate and speed, these operations are usually performed without lifting the device holding the nozzle away from the cutting position thereof.
Under these circumstances, the solution provided in JP-A-7251287 does not enable the mobile element to be moved away from the sheet, and the aforementioned problems become relevant, with the mobile element permanently protruding outside the nozzle body. This also causes a problem in the drilling phases, since the nozzle should not be too close to the sheet on account of the significant quantities of molten metal projected.
Consequently, the problem addressed is proposing a laser-beam cutting nozzle for which the aforementioned problems are significantly reduced or even eliminated, in particular a laser nozzle the movement of which over the cut sheet is significantly less disturbed by any obstacles found on the surface of said sheet, and for which the impacts resulting from the nozzle hitting said obstacles are significantly reduced in relation to the existing solutions.
Furthermore, it must be possible to implement the laser nozzle according to the present invention industrially, and the robustness and service life thereof must be better than the existing solutions, and it must not affect operation of the cutting machine on which said nozzle is installed.